1. Field of the Invention
The present invention relates to a liquid crystal display device suppressing a phenomenon in which tone representation and color representation change depending on whether a screen is viewed from the front thereof or from an oblique direction, and relates to a method of manufacturing the same.
2. Description of the Prior Art
FIG. 1 is a plan view showing a picture element part of a conventional multi-domain vertical alignment (MVA) type liquid crystal display device, and FIG. 2 is a schematic cross-sectional view showing part of a picture element of the same (Japanese Patent Publication No. 2947350, Laid-open Japanese Unexamined Patent Publication No. 2002-107730, and the like). Specifically, liquid crystal display devices include a transmissive type liquid crystal display device in which a backlight is used as a light source and in which display is performed by use of light passing through a liquid crystal panel, a reflective type liquid crystal display device in which display is performed by use of the reflection of outside light (natural light or lamplight), and a semi-transmissive type liquid crystal display device in which display is performed by use of a backlight where it is dark and by use of the reflection of outside light where it is bright. Here, a transmissive type liquid crystal display device will be described.
As shown in these FIGS. 1 and 2, an MVA type liquid crystal display device has a liquid crystal panel 40, and two polarizing plates 41a and 41b placed in such a manner that the liquid crystal panel 40 is interposed therebetween. The liquid crystal panel 40 is constituted of a pair of substrates 10 and 20 and a liquid crystal layer 30 made of vertical alignment liquid crystal (liquid crystal with negative dielectric anisotropies) filling the space between the substrates 10 and 20. Generally, thin transparent glass plates are used as the substrates 10 and 20.
As shown in FIG. 1, on one substrate 10 (on the surface thereof which faces the liquid crystal layer 30), a plurality of gate bus lines 11 horizontally extending and a plurality of data bus lines 13 vertically extending are formed at predetermined intervals, respectively. Each of a plurality of rectangular areas defined by these gate and data bus lines 11 and 13 is a picture element area. Moreover, on the substrate 10, auxiliary capacitance bus lines 12 across the centers of the respective picture element areas are formed parallel to the gate bus lines 11.
Each picture element area is provided with an auxiliary capacitance electrode 14, a thin film transistor (TFT) 15 which functions as a switching element, and a picture element electrode 16 made of transparent conductive material, such as Indium-Tin Oxide (ITO).
In this liquid crystal display device shown in FIGS. 1 and 2, part of the gate bus lines 11 are used as the gate electrodes of the TFTs 15. The drain electrode of TFT 15 is connected to the data bus line 13, and the source electrode thereof is connected to the picture element electrode 16. Moreover, the auxiliary capacitance electrode 14 is formed between the auxiliary capacitance bus line 12 and the picture element electrode 16 with first and second insulating films (both are not shown) interposed therebetween, respectively. The auxiliary capacitance electrode 14 is electrically connected to the picture element electrode 16 through a contact hole provided in the second insulating film.
In picture element electrode 16, a comb-shaped slit 16a for domain regulation is provided. This comb-shaped slit 16a includes, for example, a main slit having a width of approximately 10 μm which diagonally extends in the picture element area, and fine slits having widths of approximately 3 μm which extend in directions perpendicular to the main slit. Moreover, a vertical alignment film 18 made of polyimide is formed above the substrate 10, and the surfaces of the picture element electrodes 16 are covered with this vertical alignment film 18.
The other substrate 20 (on the surface thereof which faces the liquid crystal layer 30) is subject to formation of an opposing electrode (common electrode) 23 made of transparent conductive material, such as ITO, and domain regulation protrusions (so-called bank) 24. As shown in FIGS. 1 and 2, the domain regulation protrusions 24 are placed in portions between the slits 16a of the picture element electrodes 16 and in portions overlapping part of the edges of the picture element electrodes 16 which are positioned in the width direction of the picture element electrodes 16. The surfaces of the opposing electrode 23 and the protrusions 24 are covered with a vertical alignment film 28 made of polyimide.
Hereinafter, the substrate 10 on which the TFTs 15, the picture element electrodes 16, and the like are formed is referred to as a TFT substrate, and the substrate 20 on which the common electrode 23 and the like are formed and which is placed to be opposed to the TFT substrate is referred to as an opposing substrate.
Specifically, though not shown in FIGS. 1 and 2, a black matrix for shading areas between the picture elements and TFT-forming areas is formed on at least one of the TFT and opposing substrates. Moreover, in the case of a color liquid crystal display device, a color filter is formed on at least one of the TFT and opposing substrates.
In the liquid crystal display device thus constructed, in the state where a voltage is not applied, the liquid crystal molecules between the picture element electrode 16 and the opposing electrode 23 are aligned with the direction perpendicular to the surfaces of the substrates. In this case, light entering the liquid crystal layer 30 through the polarizing plate 41a from the bottom of the TFT substrate (substrate 10) passes through the liquid crystal layer 30 without change in the polarization direction thereof, and blocked by the polarizing plate 41b on the opposing substrate (substrate 20). That is, this case results in dark display (black display).
On the other hand, when a sufficiently high voltage is applied as a display signal to a data bus line 13 and a scan signal is supplied to a gate bus line 11, the TFT 15 is turned on, and the display signal is written to the picture element electrode 16 and the auxiliary capacitance electrode 14. Thus, the liquid crystal molecules between the picture element electrode 16 and the opposing electrode 23 are aligned with the direction perpendicular to the electric field (direction parallel to the surfaces of the substrates), and light entering the liquid crystal layer 30 through the polarizing plate 41a from the bottom of the TFT substrate changes the polarization direction thereof in the liquid crystal layer 30 to pass through the polarizing plate 41b on the opposing substrate. That is, this case results in bright display (white display). A desired image can be displayed on the liquid crystal display device by controlling a voltage applied to the picture element electrode for each picture element.
In an MVA type liquid crystal display device, as described previously, the slits 16a and the protrusions 24 are formed as domain regulation structures. Around the vicinities of these domain regulation structures, the domain regulation structures determine the directions in which the liquid crystal molecules are tilted when voltages are applied to the picture element electrodes 16. That is, since an electric field occurs in oblique directions toward the center of the slit in the edge portion of each slit 16a, the directions in which liquid crystal molecules are tilted are different from each other on opposite sides of each slit 16a. Moreover, in the state where a voltage is not applied, since liquid crystal molecules around the vicinity of protrusion 24 are aligned perpendicular to the surface of the protrusion 24, the alignment directions of the liquid crystal molecules are different from each other on both sides of the protrusion 24. Accordingly, when an electric field occurs between a picture element electrode 16 and the opposing electrode 23, liquid crystal molecules are tilted in different directions on both sides of the protrusions 24. Display performance, particularly viewing angle characteristics, is significantly improved by providing, within each picture element, a plurality of areas (multi-domain) in which alignment directions of liquid crystal molecules are different from each other. In particular, in the case where a large number of fine slits are provided in the picture element electrodes 16 as shown in FIG. 1, the control of liquid crystal molecules is substantially improved, and the effect of further improving response characteristics and viewing angle characteristics can be obtained.
Specifically, in a conventional general liquid crystal display device, after alignment films are formed on the surfaces of TFT and opposing substrates, the space between the TFT and opposing substrates is filled with liquid crystal. On the other hand, in recent years, manufacturing technology in which forming alignment films can be omitted has been proposed by the applicant of the present application. In this technology, the space between TFT and opposing substrates is filled with liquid crystal to which reactive monomers are added, and then the reactive monomers are polymerized, whereby alignment control layers for vertically aligning the liquid crystal molecules are formed on the surfaces of the substrates. Monomers which form alignment control layers for aligning liquid crystal molecules with a fixed direction by polymerization as described above are referred to as monomers having the capability of controlling alignment. Moreover, a liquid crystal display device in which alignment control layers are formed in this way is referred to as an alignment film-less type liquid crystal display device.
Incidentally, in a general liquid crystal display device, in order to specify the directions in which liquid crystal molecules are tilted when a voltage is applied to electrodes, rubbing treatment in which the surfaces of alignment films are rubbed with a cloth of nylon or the like in one direction is performed. However, rubbing treatment has problems such as the occurrence of dust and the breakdown of a TFT due to static electricity. As technology to improve these disadvantages of rubbing treatment, there has been known a method in which monomers added to liquid crystal are solidified to specify the tilt directions of the liquid crystal molecules. This method uses substrates in which vertical alignment films are previously formed on the surfaces thereof. Furthermore, a polymerization component containing bifunctional monomers as a main component is added to liquid crystal, the space between a pair of substrates is filled with the liquid crystal, and light (UV) is applied thereto in the state where a voltage is applied to electrodes, whereby the monomers are solidified to be polymerized. This makes it possible to specify the directions in which the liquid crystal molecules are tilted under the influence of the solidified polymers.
Moreover, the following is described in Laid-open Japanese Unexamined Patent Publication No. Hei 11(1999)-95221: after the space between a pair of substrates is filled with liquid crystal containing light-curing polymeric resin, ultraviolet light is applied thereto from a predetermined direction to form directional alignment films.
Each of the above-described liquid crystal display devices has alignment films. On the other hand, in an alignment film-less liquid crystal display device, alignment films need not be formed on the surfaces of substrates. Moreover, a polymerization component containing monofunctional monomers as a main component are used as a polymerization component, and light (UV) is applied in the state where a voltage is not applied to electrodes, whereby polymer layers are formed on the surfaces of substrates to vertically align liquid crystal molecules.
However, the inventors of the present application consider that the above-described conventional art has the following problem. In other words, in a conventional MVA type liquid crystal display device, there occurs the phenomenon in which tone representation and color representation significantly change depending on whether a screen is viewed from the front thereof (from the direction of the normal to a panel) or from an oblique direction. This phenomenon is called “discolor” because a screen looks whitish when viewed from an oblique direction. Note that this phenomenon occurs not only in MVA type liquid crystal display devices but also in other VA type liquid crystal display devices and twisted nematic (TN) type liquid crystal display devices.